Back to EveryPatent.com
United States Patent |
5,194,847
|
Taylor
,   et al.
|
March 16, 1993
|
Apparatus and method for fiber optic intrusion sensing
Abstract
Apparatus for sensing intrusion into a predefined perimeter comprises means
for producing a coherent pulsed light, which is injected into an optical
sensing fiber having a first predetermined length and positioned along the
predefined perimeter. A backscattered light in response to receiving the
coherent light pulses is produced and coupled into an optical receiving
fiber. The backscattered light is detected by a photodetector and a signal
indicative of the backscattered light is produced. An intrusion is
detectable from the produced signal as indicated by a change in the
backscattered light. To increase the sensitivity of the apparatus, a
reference fiber and an interferometer may also be employed.
Inventors:
|
Taylor; Henry F. (College Station, TX);
Lee; Chung E. (College Station, TX)
|
Assignee:
|
Texas A & M University System (College Station, TX)
|
Appl. No.:
|
737449 |
Filed:
|
July 29, 1991 |
Current U.S. Class: |
340/557; 340/541; 340/600; 340/666 |
Intern'l Class: |
G08B 013/10; G08B 013/18 |
Field of Search: |
340/557,541,666,600,556
|
References Cited
U.S. Patent Documents
4367460 | Jan., 1983 | Hodara | 340/550.
|
4477725 | Oct., 1984 | Asawa et al. | 250/227.
|
4482890 | Nov., 1984 | Forbes et al. | 340/666.
|
4654520 | Mar., 1987 | Griffiths | 250/227.
|
4699513 | Oct., 1987 | Brooks et al. | 356/345.
|
4759627 | Jul., 1988 | Thylen et al. | 250/227.
|
4770535 | Sep., 1988 | Kim | 356/345.
|
4848906 | Jul., 1989 | Laytoa | 356/345.
|
4928004 | May., 1990 | Zimmermann et al. | 250/227.
|
4931771 | Jun., 1990 | Kahn | 340/556.
|
4994668 | Feb., 1991 | Lagakos et al. | 250/227.
|
Other References
Griffiths, Garry D. "Fiber-Optic Sensors, Systems and Applications in
Physical Security" Pilkington PE Limited Sales Article.
|
Primary Examiner: Swann, III; Glen R.
Attorney, Agent or Firm: Baker & Botts
Claims
What is claimed is:
1. Apparatus for sensing intrusion into a predefined perimeter, comprising:
means for producing a plurality of coherent light pulses having a spectral
width less than 0.1T.sub.1, where T.sub.1 is the width of each coherent
light pulse;
an optical sensing fiber having a first predetermined length receiving at
least a portion of said plurality of coherent light pulses and being
positioned along said predefined perimeter, said optical sensing fiber
producing a backscattered light in response to receiving said plurality of
coherent light pulses;
an optical receiving fiber arranged to receive said backscattered light
from said optical sensing fiber; and
detecting means coupled to said optical receiving fiber for receiving said
backscattered light and producing a signal indicative of an intrusion in
response to a perturbation in said backscattered light.
2. The apparatus, as set forth in claim 1, wherein said coherent light
pulse producing means comprises:
a continuously operating laser producing a light; and
means coupled to said laser for modulating said light and producing pulses
of coherent light.
3. The apparatus, as set forth in claim 2, wherein said coherent light
pulse producing means further comprises isolating means coupled to said
laser for preventing optical feedback to said laser.
4. The apparatus, as set forth in claim 2, wherein said coherent light
pulse producing means includes an optical switch.
5. The apparatus, as set forth in claim 2, wherein said coherent light
pulse producing means includes an optical intensity modulator.
6. The apparatus, as set forth in claim 2, wherein said coherent light
pulse producing means includes an optical amplifier operating in a pulsed
mode.
7. The apparatus, as set forth in claim 1, further comprising a reference
fiber having a predetermined length substantially equal to said first
predetermined length of said optical sensing fiber and receiving at least
a portion of said coherent pulsed light from said coherent pulsed light
producing means and producing a backscattered light.
8. The apparatus, as set forth in claim 7, wherein said reference fiber has
a non-reflecting end.
9. The apparatus, as set forth in claim 7, wherein said reference fiber is
protected from refractive index perturbations.
10. The apparatus, as set forth in claim 7, wherein said optical receiving
fiber is optically coupled to both said optical sensing fiber and said
reference fiber to receive backscattered light therefrom.
11. The apparatus, as set forth in claim 1, wherein said optical sensing
fiber terminates in a non-reflecting end.
12. The apparatus, as set forth in claim 1, further comprising means for
optically coupling said optical sensing fiber and said optical receiving
fiber.
13. The apparatus, as set forth in claim 12, Wherein said optical coupling
means includes an optical switch.
14. The apparatus, as set forth in claim 12, wherein said optical coupling
means includes a fiber coupler.
15. The apparatus, as set forth in claim 1, wherein said detecting means
comprises a photodetector coupled to said optical receiving fiber for
receiving said backscattered light and producing an electrical signal
indicative of the optical power of said backscattered light.
16. The apparatus, as set forth in claim 15, wherein said detecting means
further comprises means coupled to said photodetector for amplifying said
electrical signal.
17. The apparatus, as set forth in claim 1, further comprising an
interferometer coupled to said optical receiving fiber for optically
mixing said backscattered light.
18. The apparatus, as set forth in claim 1, further comprising a
Mach-Zehnder interferometer with unequal path lengths coupled to said
optical receiving fiber for mixing said backscattered light.
19. The apparatus, as set forth in claim 1, further comprising:
second coupling means coupled to said optical receiving fiber;
third coupling means coupled to said second coupling means via a first and
second fiber arm, the lengths of said first and second fiber arms being
unequal, said received backscattered light traveling along said first and
second fiber arms and mixing coherently at said third coupling means; and
said detecting means coupled to said third coupling means and receiving
said optically mixed light from said interferometer, and producing an
electrical signal indicative of an intrusion in response to a change in
said optically mixed light.
20. The apparatus, as set forth in claim 19, wherein said detecting means
comprises:
two photodetectors coupled to said third coupling means and producing two
electrical signals indicative of the optical power of light in said
optically mixed light; and
a differential amplifier receiving said two electrical signals and
producing an output signal indicative of their difference.
21. The apparatus, as set forth in claim 19, wherein said electrical signal
is indicative of an intrusion at a distance L.sub.i along said first
predetermined length of said optical sensing fiber computable by:
##EQU7##
where T.sub.i is the time delay associated with said perturbation, c is
the free-space velocity of light, and n.sub.g is the group refractive
index of said optical sensing fiber.
22. A method for sensing intrusion into a predefined perimeter, comprising
the steps of:
producing a coherent pulsed light having a spectral width less than
0.1T.sub.1, where T.sub.1 is the width of each coherent light pulse;
injecting at least a portion of said coherent pulsed light into an optical
sensing fiber having a first predetermined length and positioned along
said predefined perimeter;
producing a backscattered light in response to receiving said coherent
pulsed light; and
receiving said backscattered light from said optical sensing fiber and
producing a signal indicative of an intrusion in response to a
perturbation in said backscattered light.
23. The method, as set forth in claim 22, wherein said coherent pulsed
light producing step comprises the steps of:
operating a laser continuously and producing light; and
receiving said light and producing a plurality of coherent light pulses.
24. The method, as set forth in claim 23, wherein said coherent light pulse
producing step further includes the step of preventing optical feedback to
said laser.
25. The method, as set forth in claim 23, wherein said coherent light pulse
producing step includes operating an optical switch.
26. The method, as set forth in claim 23, wherein said coherent light pulse
producing step includes operating an optical intensity modulator.
27. The method, as set forth in claim 23, wherein said coherent light pulse
producing step includes operating an optical amplifier in a pulsed mode.
28. The method, as set forth in claim 22, further comprising the steps of:
further injecting at least a portion of said coherent pulsed light into a
reference fiber having a predetermined length substantially equal to said
first predetermined length of optical sensing fiber;
producing a backscattered light therefrom; and
receiving and mixing said backscattered light from said optical sensing
fiber and said reference fiber.
29. The method, as set forth in claim 22, further comprising the step of
optically coupling said optical sensing fiber and said optical receiving
fiber.
30. The method, as set forth in claim 29, wherein said coupling step
includes operating an optical switch, opening said optical switch to
deliver said coherent pulsed light to said optical sensing fiber and
closing said optical switch to deliver said backscattered light to said
optical receiving fiber.
31. The method as set forth in claim 29, wherein said coupling step
includes providing a fiber coupler therebetween.
32. The method, as set forth in claim 31, further comprising the steps of:
receiving said backscattered light;
splitting said backscattered light into a first and second portion;
introducing a predetermined amount of time delay to said second portion;
coherently mixing said first and delayed second portions of backscattered
light;
detecting an interference pattern; and
producing a signal indicative of said interference pattern.
33. The method, as set forth in claim 23, wherein said receiving step
further comprises the step of photodetecting said backscattered light and
producing an electrical signal indicative of the optical power of said
backscattered light.
34. The method, as set forth in claim 23, wherein said receiving step
further comprises the step of amplifying said electrical signal.
35. The method, as set forth in claim 22, further comprising the step of
computing for a distance L.sub.i along said optical sensing fiber where
said intrusion took place with:
##EQU8##
where T.sub.i is the time delay associated with said perturbation, c is
the free-space velocity of light, and n.sub.g is the group refractive
index of said optical sensing fiber.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of intrusion sensors. More
particularly, the present invention relates to apparatus and a method for
fiber optic intrusion sensing.
BACKGROUND OF THE INVENTION
Intrusion sensors are widely used in security systems to monitor the
boundaries of a well defined area in order to detect the presence,
location and motion of people and vehicles. Exemplary sites that may
benefit from the use of such security systems are national borders,
boundaries of military installations, nuclear power plants, prisons, and
businesses. A number of existing intrusion sensors based on seismic,
ultrasonic, infrared, and magnetic technologies have been employed, but
are unfavorable for numerous reasons. These existing systems are
expensive, difficult to conceal, have high false alarm rates, and are
capable of providing coverage for only a limited portion of a perimeter.
By way of illustration, in the particular application of intrusion sensing
to the monitoring of national borders to detect and apprehend illegal drug
smugglers or illegal aliens, the border terrain under surveillance may be
vast and rugged. The sensor system employed in this application must not
only detect an intrusion but must also be able to determine the location
of the intrusion along the monitored border. The sensor system must also
be easily concealable to prevent tampering. Furthermore, such a system
must have and maintain a record of low false alarms. Conventional
intrusion sensing systems are not able to meet such stringent requirements
or cannot provide satisfactory performance without incurring prohibitive
costs.
Accordingly, it is desirable to provide an intrusion sensing system which
provides intrusion detection as well as the location of intrustion. It is
further desirable that such sensing system be deployable in a reasonable
manner over a vast area, such as for the purpose of monitoring a national
border.
SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus and a method for
intrusion sensing are provided which substantially eliminate or reduce
disadvantages and problems associated with prior systems.
In one aspect of the present invention, apparatus for sensing intrusion
into a predefined perimeter is provided. The intrusion sensing apparatus
comprises a coherent light pulse source injecting coherent light pulses
into an optical sensing fiber having a first predetermined length and
positioned along the predefined perimeter. Light is backscattered from the
optical sensing fiber and coupled into an optical receiving fiber. The
backscattered light is detected by a photodetector coupled to the optical
receiving fiber and a signal is produced in response thereto. An intrusion
is detectable as a change in the produced signal.
In another aspect of the present invention, apparatus for sensing intrusion
into a predefined border is provided. The apparatus includes a coherent
light pulse source injecting coherent light pulses into an optical sensing
fiber placed along the border and having a predetermined length. An
optical receiving fiber is optically coupled to the optical sensing fiber
and receives a backscattered light therefrom. A fiber optic interferometer
with unequal path lengths is coupled to the optical receiving fiber and
produces a modification of the backscattered light due to optical
interference. A photodetector coupled to the interferometer produces a
signal in response thereto. An intrusion is detectable as a change in the
produced signal.
In yet another aspect of the present invention, apparatus for intrusion
sensing includes a coherent light pulse source producing a plurality of
coherent light pulses. A sensing fiber placed along a border being
monitored receives the coherent light pulses and produces backscattered
light. A reference fiber also receives the coherent light pulses and
produces a backscattered light. The backscattered light from both fibers
is allowed to mix coherently to produce an interference pattern which is
received by a photodetector. A change in the photodetector signal is
indicative of an intrusion.
In another aspect of the present invention, a method for sensing intrusion
into a predefined perimeter is provided. The method comprises the steps of
producing a plurality of coherent light pulses and delivering the
plurality of coherent light pulses into an optical sensing fiber having a
first predetermined length and being positioned along the predefined
perimeter. Backscattered light is produced in the optical sensing fiber
and a signal indicative of the backscattered light is generated. An
intrusion is indicated by a changed in the signal.
In still another aspect of the present invention, an intrusion sensing
method is provided which injects coherent light pulses into a length of
sensing fiber placed along the border being monitored. Backscattered light
is produced and injected into a fiber optic interferometer with unequal
path lengths. Light from the interferometer is then detected by a
photodetector which produces a signal that indicates an intrusion when a
change in the interference pattern occurs.
In another aspect of the present invention, an intrusion sensing method
provides, in addition, the step of injecting the plurality of coherent
light pulses into a reference fiber, which also produces backscattered
light that is also injected into the receiving fiber. The backscattered
light from the sensor and reference fibers are allowed to mix coherently
to produce an interference pattern. Thereafter the light is detected and
serves to indicate an intrusion when a change or perturbation in the
photodetector signal occurs.
A further important technical advantage is the concealability, versatility
and relatively low cost of the present intrusion sensor.
An important technical advantage of the present invention provides an
intrusion sensing apparatus capable of monitoring a vast area or a
distance extending several kilometers or tens of kilometers.
Another important technical advantage provides an intrusion sensing
apparatus which can detect an intrusion as well as determine substantially
the location of the intrusion along the monitored border or perimeter.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, reference may be made
to the accompanying drawings, in which:
FIG. 1 is a simplified schematic drawing of a conventional optical
time-domain reflectometer (OTDR) used in the telecommunications industry;
FIG. 2 is a reflected power vs. time graph of an output from the
conventional OTDR;
FIG. 3 is a simplified schematic of an embodiment of intrusion sensing
apparatus constructed in accordance with the present invention;
FIG. 4 is a simplified schematic of an embodiment of the intrusion sensing
apparatus constructed in accordance with the invention and using a
reference fiber;
FIG. 5 is a simplified schematic of a preferred embodiment of the intrusion
sensing apparatus constructed in accordance with the invention and
incorporating a Mach-Zehnder interferometer;
FIG. 6 is a simplified schematic of a preferred embodiment of the intrusion
sensing apparatus constructed in accordance with the invention and using
an optical switch;
FIG. 7 illustrates the operating states of the optical switch used in
apparatus of FIG. 6;
FIG. 8 is a simplified schematic of another embodiment of the intrusion
sensing apparatus constructed in accordance with the present invention;
FIG. 9 is an optical power vs. time graph of the coherent pulsed light
input used in the intrusion sensing apparatus;
FIG. 10 is a graphical depiction of an exemplary output signal from the
intrusion sensing apparatus; and
FIG. 11 is a graphical depiction of an exemplary output signal from the
intrusion sensing apparatus when an intrusion is detected.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawings, FIG. 1 illustrates a conventional optical
time-domain reflectometer (OTDR) 10 commonly used in the
telecommunications industry to detect breakage in fiber optic cables. A
semiconductor laser 12 produces light pulses of low coherence which are
injected into a fiber 14. The spectral width of light pulses generated by
laser 12 may be in the range of 100 MHz to 10 GHz. A fiber coupler 16
optically couples fiber 14 with another fiber 20, so that a portion of the
backscattered light from fiber 18 produced by a phenomenon called Rayleigh
backscattering is injected into fiber 20. A photodetector 22 is coupled to
fiber 20 to receive the backscattered light and produces an electrical
signal which is displayable by an oscilloscope 24.
FIG. 2 depicts an exemplary output waveform representative of the power of
the backscattered light, as detected by photodetector 22, versus time.
Discontinuities 28 and 30 in the waveform such as shown in FIG. 2 indicate
the presence and location of breaks (28) or localized attenuation (30) in
fiber 18. Conventional OTDR systems 10 perform adequately for detecting
breaks in telecommunications fiber optics cables, but are ill-suited to
intrusion sensing applications because they are not sensitive enough to
either acoustic waves or pressure.
Referring to FIG. 3, a simplified schematic of an embodiment 40 of
apparatus for intrusion sensing constructed in accordance with the present
invention is shown. A continuous, single mode laser 42 functions as the
source of coherent light. Laser 42 could be a semiconductor laser diode, a
solid state laser such as a neodymium yttrium aluminum garnet (Nd:YAG)
laser or other lasers which produce a suitably narrow spectral line. A
laser diode used on laser 42 preferably has an external cavity (not shown)
to achieve line narrowing or a high degree of spectral purity. The
spectral width of the laser output from laser 42 is preferably of the
order of 10 kHz. An optical isolator 44 may be coupled to the output of
laser 42 to prevent destabilization of the laser spectral purity due to
optical feedback from the rest of the system. The coherent light passes
through optical isolator 44 and enters an optical intensity modulator 46,
which is operating in a pulsed mode to produce short pulses of coherent
light. Optical intensity modulator 46 can be of acousto-optic or
electro-optic design, either of bulk optical or integrated optic
construction. Alternatively, optical intensity modulator 46 can be an
all-fiber acousto-optic device. Still another alternative is to use a
pulsed semiconductor optical amplifier as optical intensity modulator 46,
in which case the light from laser 42 is amplified as well as modulated.
Light pulses of high spectral purity are then injected into a fiber 48.
FIG. 9 is a graph illustrating the optical power of the coherent light
pulses versus time, where the width of each pulse is T.sub.1 and the time
between pulses is T.sub.2. For example, T.sub.1 can be approximately 100
nsec and T.sub.2 can be approximately 100 .mu.sec. The derivation and
computation of T.sub.1 and T.sub.2 are discussed below. The spectral width
of laser 42 is preferably as narrow as possible. More specifically, the
spectral width is related to T.sub.1 such that the spectral width is less
than 0.1T.sub.1 and is preferably less than 0.01T.sub.1.
A fiber coupler is a conventional device typically having two arms on each
side, where if light enters the coupler carried on one arm on a first
side, the light exits the coupler equally distributed on the two arms on
the second side, and vice versa. Such a fiber coupler is known as a "3 dB"
device. Fiber 48, which is coupled to optical intensity modulator 46 at
one end, is coupled to the first side of a fiber coupler 50 at the other.
A sensing fiber 52 terminating in a non-reflecting end 54 is coupled to
the second side of fiber coupler 50. Sensing fiber 52 is preferably
encased in a fiber cable (not shown) and may be as long as 50 km due to
the relatively low loss of optical fibers. A fiber 56 also with a
non-reflecting end 58 is connected to fiber coupler 50 on the same side.
Another fiber 60 is coupled to fiber coupler 50 on the first side and
optically connected to sensing fiber 52 by fiber coupler 50. The other end
of fiber 60 is coupled to a photodetector 62, which is coupled to an
electronic amplifier 64.
Therefore, coherent light pulses are coupled into sensing fiber 52 by fiber
coupler 50 and light backscattered from sensing fiber 52 due to Rayleigh
backscattering is coupled into fiber 60, which is then received by
photodetector 62. Photodetector 62 converts the light energy into an
electrical signal and supplies it to amplifier 64. Amplified output signal
66 is a seemingly random signal plotted against time, also shown in FIG.
10. Signal 67 in FIG. 10 represents a moving-time-window interference
pattern for light backscattered from sensing fiber 52. This pattern
represents the interference of backscattered light from different parts of
sensing fiber 52 which arrive at photodetector 62 at the same time. If
sensing fiber 52 is subjected to impinging acoustic waves or to pressure,
a localized change in the effective refractive index of sensing fiber 52
is induced. The change will occur at a time corresponding to the location
of the intrusion along sensing fiber 52. FIG. 11 illustrates signal 67
showing the effect of localized phase perturbation as a spike 68 in signal
67. The change in the detected signal depicted in FIG. 11 results from the
different relation of the phases of the light backscattered from portions
of sensing fiber 52 induced by localized phase perturbation. The temporal
extent of the change is limited to the temporal characteristics of the
optical pulse injected into sensing fiber 52.
In operation, sensing fiber 52 may be buried one to two feet underground to
detect foot or vehicular traffic, placed underwater to detect powered
boats or swimmers, or aerially or above ground to detect low-flying
aircraft. The impinging acoustic waves or pressure on sensing fiber 52
causes a change in the fiber's effective refractive index. This change is
manifested in the backscattered light from sensing fiber 52 caused by
Rayleigh backscattering, which serves as the optical carrier. The
backscattered light is passed through fiber coupler 50 into fiber 60, and
then detected by photodetector 62. The output from photodetector 62 is
then amplified by amplifier 64.
When sensing fiber 52 is disturbed by either pressure or acoustic waves,
the backscattered light is altered at a time corresponding to the location
of the disturbance. Using digital signal processing methods, the amplified
signal may be digitized and processed in a time-resolved manner, such as
those used in pulsed-radar signal processing. For example, the
backscattered signal is divided and grouped into a number of time bins
according to the time delay thereof, where each time bin corresponds to a
length of sensing fiber 52 located at a particular distance from fiber
coupler 50 along sensing fiber 52. A phase change in the backscattered
signal in a time bin would indicate some traffic across the perimeter
being monitored at the location corresponding to that time bin. Additional
signal processing may be used to increase the sensitivity of the system
and further minimize false alarm rates. Signature analysis may also be
used to identify the type of intruder, i.e., to distinguish between
humans, vehicles, and animals.
Referring to FIG. 4, another embodiment 70 of the present invention is
shown. Embodiment 70 is substantially similar to embodiment 40 shown in
FIG. 3, and includes a single mode, continuously operating laser 72
coupled to an optical isolator 74 and a pulsed optical intensity modulator
76. The coherent pulsed light output from optical intensity modulator 76
is injected into a fiber 78, which is coupled to a fiber coupler 80. Fiber
coupler 80 couples fiber 78 to a sensing fiber 82 of a predetermined
length L.sub.s, terminating in a non-reflecting end 84. A reference fiber
86, having a non-reflecting end 88 and protected from refractive-index
perturbations, is also optically coupled to fiber 78 by fiber coupler 80.
The length of reference fiber 86 is comparable to L.sub.s, or the length
of sensing fiber 82. A fiber 90 is coupled to the receiving side of fiber
coupler 80 and is further connected to a photodetector 92. Photodetector
92 is coupled to an amplifier 94.
In operation, fiber coupler 80 optically couples fiber 78 to sensing fiber
82 and reference fiber 86 to allow coherent light pulses of approximately
equal intensity to travel down both fibers 82 and 86. Light backscattered
from both sensing fiber 82 and reference fiber 86 mixes at fiber coupler
80, after which it is detected by photodetector 92. Amplifier 94 then
amplifies the signal output from photodetector 92. In this arrangement, a
localized change in the effective refractive index of sensing fiber 82,
indicative of an intrusion, affects output signal 96 in not only the time
bin associated with the position of intrusion, but also the output signal
in all time bins subsequent thereto.
Referring to FIG. 5, a preferred embodiment 100 of the present invention
using a Mach-Zehnder interferometer is shown. Embodiment 100 includes a
single mode, continuously operating laser 102 which produces light of a
high degree of spectral purity. This coherent light is received by an
optical isolator 104 and pulsed by an optical intensity modulator 106, as
described above. The coherent pulses of light are injected into a fiber
108, which is coupled to a first fiber coupler 110. A sensing fiber 112 of
length L.sub.s, having a non-reflecting end 114, is optically coupled to
fiber 108 by first fiber coupler 110. A fiber 116, also having a
non-reflecting end 118, is also optically coupled to fiber 108 by fiber
coupler 110. A fiber 120 connects first fiber coupler 110 to a second
fiber coupler 126. Second fiber coupler 126 also has a fiber 122 with a
non-reflecting end 124 coupled thereto. Fiber coupler 126 optically
connects fiber 120 with two fibers 128 and 130 which are further coupled
to a third fiber coupler 132.
Fibers 128 and 130 form the two arms of a Mach-Zehnder interferometer 131
as known in the art, which also includes second fiber coupler 126 and
third fiber coupler 132. Fibers 128 and 130 are of unequal lengths
represented by L.sub.a and L.sub.b, respectively. Two photodetectors 134
and 136 are coupled to the two outputs from Mach-Zehnder interferometer
131. A differential amplifier 138 is coupled to the outputs of
photodetectors 134 and 136 and produces an output signal 140 that may be
further analyzed by digital signal processing methods as known in the art.
Intrusion sensing apparatus 100 shown in FIG. 5 probably has the highest
sensitivity of contemplated embodiments described above and below through
the use of Mach-Zehnder interferometer 131. In operation, the
backscattered light from sensing fiber 112 passes through fiber coupler
110 and is injected into fiber 120. As light passes through fiber coupler
126, both arms 128 and 130 of interferometer 131 receive approximately
half of the light in fiber 120. The light passing through arms 128 and 130
then mixes coherently at fiber coupler 132. At any instant in time, the
light arriving at fiber coupler 132 through arm 128 having length L.sub.a
is the light that was backscattered from a point in sensing fiber 112 at a
distance (L.sub.b -L.sub.a)/2 downstream from the light arriving at
coupler 132 through arm 130 of length L.sub.b. Optical outputs from
interferometer 131 are detected by two photodetectors 134 and 136 and
subtracted in differential amplifier 138.
Output signal 140 from differential amplifier 138 represents a
moving-time-window interference pattern for light backscattered from
locations in sensing fiber 112 separated by a distance (L.sub.b
-L.sub.a)/2, and varies with time in an apparently random fashion. A
localized change in the effective refractive index of sensing fiber 112
indicative of an intrusion causes a change in output signal 140 occurring
at a time corresponding to the intruder's location along sensing fiber
112. However, unlike implementation 70 shown in FIG. 4, a localized
perturbation in the effective refractive index of sensing fiber 112,
indicative of an intrusion, affects output signal 140 in only the time bin
associated with the position of intrusion and not in all subsequent time
bins. In particular, the time delay T.sub.i corresponding to an intruder
located a distance L.sub.i along sensing fiber 112 can be expressed as:
##EQU1##
where c is the free-space velocity of light and n.sub.g is the group
refractive index of sensing fiber 112. Alternatively, the distance L.sub.i
can be expressed as:
##EQU2##
Typically, n.sub.g is approximately 1.46 for silica fiber. Computing for
T.sub.i using equation (1), if L.sub.i is equal to 1 km, then T.sub.i is
equal to 9.7 .mu.sec, where c is equal to 3.times.10.sup.8 m/sec.
Further consideration must be given to the spatial resolution of the output
signal. Spatial resolution S is related to the light pulse width T.sub.1
(shown in FIG. 9) and can be expressed as:
##EQU3##
It can be appreciated from equation (3) that better spatial resolution can
be achieved by a shorter light pulse input. For example, if T.sub.1 is
equal to 100 nsec, then S is less than 10.3 m. The spatial resolution is
also related to the lengths L.sub.a and L.sub.b of arms 128 and 130,
respectively, of Mach-Zehnder interferometer 131 in the following manner:
##EQU4##
Thus, S is less than 10 m if the quantity (L.sub.b -L.sub.a) is equal to
20 m.
In general, it is preferable to have:
##EQU5##
so that if the light pulse width T.sub.1 equals 100 nsec, then the
quantity (L.sub.b -L.sub.a) is approximately 20 m, a preferable value for
the difference of arm lengths 128 and 130. The time T.sub.2 between the
pulses, as shown in FIG. 9, is related to sensor fiber length L.sub.s and
can be expressed by the following:
##EQU6##
Therefore, if the sensor fiber length L.sub.s is 10 km, then T.sub.2 is
greater than 97 .mu.sec.
Referring to FIG. 6, an alternate embodiment 150 also using a Mach-Zehnder
interferometer 151 is shown. The sensing apparatus comprises a coherent
light pulse generator, which includes a single mode, continuously
operating laser 152 isolated from feedback from the rest of the apparatus
by an optical isolator 154, and a pulsed optical intensity modulator 156.
The coherent light pulses are injected into and carried by a fiber 158
which is optically coupled to a sensing fiber 162 by an optical switch
160. Sensing fiber 162 is an optical fiber encased in a cable, and has a
length L.sub.s and terminates in a non-reflecting end 164. Sensing fiber
162 is placed along a border where intrusion sensing is desired. A fiber
166, also with a non-reflecting end 168, may be coupled to optical switch
160. A fiber 170 is optically coupled to sensing fiber 162 by optical
switch 160 and leads into Mach-Zehnder interferometer 151. Interferometer
151 consists of two fibers 178 and 180 having lengths L.sub.a and L.sub.b,
respectively, connected between two fiber couplers 176 and 182. The
outputs of interferometer 151 are supplied to photodetectors 184 and 186,
the outputs of which are then supplied to a differential amplifier 188. A
possible waveform of the output signal 190 of differential amplifier 188
is shown.
Basically, intrusion sensing apparatus 150 operates in a similar fashion to
apparatus 100 of FIG. 5. However, fiber coupler 110 of apparatus 100 is
replaced by optical switch 160. Because a 3 dB loss is experienced when
light passes through a fiber coupler in both the forward and reverse
directions, a total loss of 6 dB can be eliminated by substituting optical
switch 160 in place of fiber coupler 110. Referring also to FIG. 7, the
two-state operation of optical switch 160 of FIG. 6 is shown. In the first
state of operation, fiber 158 is coupled to sensing fiber 162 without
interference from fiber 166 to allow coherent light pulses from pulsed
optical intensity modulator 156 to reach sensing fiber 162 without
substantial loss. In the second state of operation, sensing fiber 162 is
coupled to fiber 170, so that the backscattered light may enter fiber
coupler 176 of interferometer 151 without substantial loss. Constructed in
this manner, virtually no loss is experienced, thus making more efficient
use of the optical power in the coherent light pulses. Optical switch 160
may be an electro-optic or acousto-optic integrated optic device, both
commercially available, or an acousto-optic fiber device.
Referring presently to FIG. 8, another arrangement 200 of the intrusion
sensing apparatus is shown. A narrow line source such as a single mode,
continuously operating laser 202 is coupled to an optical isolator 204.
The optical output from optical isolator 204 is injected into a fiber 206,
which is optically coupled by a fiber coupler 208 with fibers 210 and 211.
A fiber 207 With a non-reflecting end 209 is coupled to fiber coupler 208.
The other end of fiber 211 is coupled to a pulsed optical intensity
modulator 212. Fiber 214 connects the output of pulsed optical intensity
modulator 212 to a second fiber coupler 216. A sensing fiber 218 extends
from fiber coupler 216 and terminates in a non-reflecting end 220. A fiber
219 with a non-reflecting end 221 is coupled to the same side of fiber
coupler 216. Fiber coupler 216 optically couples sensing fiber 218 with
yet another fiber 222. Fibers 210 and 222 are then coupled to a third
fiber coupler 224, the other side of which is connected to a pair of
photodetectors 226 and 228. The outputs of photodetectors 226 and 228 are
then supplied to a differential amplifier 230. The output of differential
amplifier 230 is a signal 232 indicative of an intrusion into the
monitored boundary.
Intrusion sensor 200 functions by allowing interference to occur between
backscattered light caused by Rayleigh backscattering from sensing fiber
218 and the light produced by the light source, laser 202 and optical
isolator 204, at fiber coupler 224 through fiber 210. The interference
effect is detected by photodetectors 226 and 228 and processed by
differential amplifier 232 as described above in conjunction with other
embodiments. A localized change in the effective refractive index of
sensing fiber 218 would cause a change in the interference pattern of the
light, which is detectable by photodetectors 226 and 228. Such change
indicates the occurrence of an intrusion, the approximate location of
which can be computed as described above.
The main components from which the present invention may be constructed are
commercially available. The following lists such commercially available
parts manufactured by British Telecom and Du Pont Technologies of
Wilmington Del. and their respective part numbers:
______________________________________
Model Number Part Name
______________________________________
TSL1000-1550 Tunable External Cavity
Semiconductor Laser
SOA1100-1550 Semiconductor Optical
Amplifier
OIC1100-1550 Optical Isolator
IOC2000-1550 Optical Integrity Modulator
______________________________________
Additionally the following parts, part numbers and their manufacturer:
______________________________________
Part Name Part Number Manufacturer
______________________________________
PINFET optical
LDPF0012 Laser Diode Inc.
receiver New Brunswick,
(photodetector) New Jersey
Fiber Coupler
SMC-08-50-2-A-1-S
Aster
Milford,
Massachusetts
Fiber Cable 001-R14-11003-20
Siecor
Hickory,
North Carolina
______________________________________
It must be emphasized that the above listings of parts are but exemplary
components that may be used in constructing the present invention, and one
skilled in the art will appreciate that the invention is not necessarily
so limited. The above-listed components implement specifically a light
wavelength of 1550 nm. It is also apparent that the continuous coherent
light beam from the laser may be converted into the coherent light pulses
shown in FIG. 9 by the pulsed optical intensity modulator shown in the
various embodiments, or by any device that may effectively and alternately
allow the light to pass and not pass in a somewhat precise manner. One
such device is the optical switch or modulator which may be an integrated
optic device. With the current laser technology, such coherent light
pulses cannot be achieved by turning the laser on and off, since the
frequency of the laser output would change due to thermally induced
"chirping" effects. It follows then that any narrow line source that is
capable of emitting coherent pulses of light such as that shown in FIG. 9
may be incorporated into the present invention.
Although the present invention has been described in detail, it should be
understood that various changes, substitutions and alterations can be made
thereto without departing from the spirit and scope of the present
invention as defined by the appended claims.
Top